1. Field of the Invention
The present invention relates to an amplifier, and more specifically to an amplifier used in a driving circuit for driving a semiconductor laser or an optical modulator.
2. Description of the Related Art
Traveling Wave Amplifier (TWA) has been used as a wideband amplifier suitable for the driving circuit. A basic circuit structure of TWA includes at least an input terminal, an input-side transmission line, a plurality of sub-amplifiers, an output-side transmission line, and an output terminal. The input terminal receives an input signal from an outside of TWA. One end of the input-side transmission line is connected to the input terminal and transmits the input signal to the sub-amplifiers. The sub-amplifiers are arranged in a line along the input-side transmission line and the output-side transmission line. Each sub-amplifier has an input node and an output node. The input nodes of the sub-amplifiers receive the input signal through the input transmission line. Each sub-amplifier amplifies the input signal and outputs an amplified signal from the output node thereof to the output-side transmission line. The output-side transmission line transmits the amplified signals to the output terminal. The amplified signals are superimposed with each other during the transmission to generate an output signal from the output terminal.
Each sub-amplifier receives the input signal from the input terminal to the input node thereof with an individual delay time, which is different from the delay time of other sub-amplifiers. Each amplified signal transmits from the output node thereof to the output terminal with an individual delay time, which is different from the delay time of other sub-amplifiers. However, sum of the delay time for the input signal and the delay time for the output signal is balanced for all the sub-amplifiers.
The sub-amplifiers may be the same single-ended amplifier to each other or the same differential amplifier to each other except a combination of them. The single-ended amplifier has an input node and an output node as described above. The differential amplifier, however, has at least a pair of input terminals and a pair of output terminals. For example, the differential amplifier receives a positive-phase input signal in one of the input nodes and a negative-phase input signal in the other of the input nodes. The positive-phase input signal and the negative-phase input signal have opposite phases to each other. In addition, the differential amplifier outputs a positive-phase output signal from one of the output nodes thereof and a negative-phase output signal from the other of the output nodes thereof. The positive-phase output signal and the negative-phase output signal have opposite phases to each other. Accordingly, in a case that differential amplifiers are used for the sub-amplifiers, the circuit configuration of the TWA needs at least a pair of input terminals, a pair of input-side transmission lines, a pair of output-side transmission lines, and a pair of output terminals except the sub-amplifiers.
As aforementioned, the input-side transmission line transmits the input signal and the output-side transmission line transmits the amplified signals output from the sub-amplifiers, respectively. In general, electrical characteristics of a transmission line can be characterized by characteristic impedance. Characteristic impedance of the input-side transmission line is often set to match with output impedance of an upstream circuit that provides the input signal to the TWA. Characteristics impedance of the output-side transmission line is often set to match with input impedance of a downstream circuit that is driven by the output signal of the TWA. The characteristic impedance depends on distributed inductor and distributed capacitor of the transmission line. The transmission lines cause respective individual delay times for a transmitted signal. Wideband amplifiers are often needed to maintain fine matching between the output impedance thereof and characteristic impedance of the transmission line up to a high frequency.